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Sludge Treatment & Biosolids Engineering

Definitions & Key Terms

  • Sludge – mixture of water and solids (concentrated impurities) separated from wastewater.
    • Sometimes called “residuals.”
  • Biosolids – treated/stabilised sludge suitable for reuse; goal is to turn “sludge” ➜ “biosolids.”
    • Webster: “solid organic matter recovered from a sewage-treatment process, used as fertiliser.”
    • McGraw-Hill: “recyclable, primarily organic solid material produced by wastewater treatment.”
  • Putrescible – liable to decay, produce odour, contain pathogens; hence stabilisation is required before disposal.
  • Screenings / Grit / Scum – coarse solids, heavy inorganics, or floatables removed in preliminary treatment.

Why Sludge Treatment Matters

  • Higher treatment levels → larger sludge volumes.
  • Sludge handling ≈ \sim 50\% of total WWTP operating cost.
  • Treatment aims: volume reduction, pathogen destruction, odour control, vector attraction reduction, resource recovery.

Typical Sludge Types & Descriptions

  • Screenings – large organic/inorganic debris captured on bar racks.
  • Grit – heavy inorganic particles settling rapidly.
  • Scum/Grease – floatables skimmed from tanks.
  • Primary Sludge – grey, slimy, odorous; 4!\text{–}6\% TS, 60!\text{–}80\% VSS.
  • Waste-Activated Sludge (WAS) – brown floc; 70!\text{–}80\% VSS, 0.5!\text{–}2\% solids.
  • Trickling-Filter (Humus) Sludge – brownish, inoffensive when fresh.
  • Digested Biosolids
    • Aerobic: brown, flocculent, musty odour, dewater easily.
    • Anaerobic: dark brown/black, gassy; when well-digested smell like burnt rubber.
  • Chemical Precipitation Sludge – dark/gelatinous; colour changes w/ precipitant (Fe red, lime grey-brown).

Solids Generation & Treatability (per \text{m}^3 of wastewater)

| Treatment stage | Volume (L) | TS % | Organic % | Relative treatability |
| Primary | 2.5!\text{–}3.5 | 3!\text{–}7 | 60!\text{–}80 | easy |
| Secondary bio | 15!\text{–}20 | 0.5!\text{–}2 | 50!\text{–}60 | difficult |
| Tertiary chem | 25!\text{–}30 | 0.2!\text{–}1.5 | 35!\text{–}50 | difficult |

US Regulatory Framework

  • Title 40 CFR Part 503 – “Standards for the Use & Disposal of Sewage Sludge.”
    1. General provisions
    2. Land application
    3. Surface disposal
    4. Pathogen & vector attraction reduction
    5. Incineration
  • Permits required for TWTDS; will be folded into NPDES.
  • Pathogen-reduction tiers
    • PFRP (Process to Further Reduce Pathogens) → Class A.
    • PSRP (Process to Significantly Reduce Pathogens) → Class B.

Biosolids Classes

Class A

  • Pathogens below detection.
    • \text{Fecal coliform} <1000\,\text{MPN g}^{-1}\,\text{TS} OR \text{Salmonella} <3\,\text{MPN}/4\,\text{g TS}.
  • No site restrictions.
  • Six alternative compliance routes (time–temperature, pH-lime, analytical verification, PFRP list, equivalent process, etc.).

Class B

  • Pathogens reduced but not eliminated; site management restrictions apply.
  • Three compliance options: fecal monitoring, PSRP process, or equivalent.

Example PFRP / PSRP Processes

PFRPKey condition
Composting\ge 55^\circ\text{C} for \ge 3 d (in-vessel/static) or 15 d w/ 3 turns (windrow)
Heat drying\le 10\% moisture, hot gases contact
Heat treatment>180^\circ\text{C}, 30\,\text{min}
Thermophilic aerobic dig.\text{SRT}=10\,\text{d}, 55!\text{–}60^\circ\text{C}
Beta/Gamma irradiation\ge 1\,\text{Mrad}
Pasteurisation70^\circ\text{C}, 30\,\text{min}
PSRPCondition
Aerobic dig.40!\text{–}60^\circ\text{C}, 40!\text{–}60\,\text{d SRT}
Anaerobic dig.15\,\text{d}@35!\text{–}55^\circ\text{C} OR 60\,\text{d}@20^\circ\text{C}
Air drying>3 mo on beds
Lime stabilisationpH>12 for \ge 2 h

Core Sludge-Treatment Train

  1. Thickening – raise solids (target \le 10\% to remain pumpable).
  2. Stabilisation/Digestion – reduce pathogens, odour, VSS.
  3. Conditioning – improve dewatering (chemical or physical).
  4. Dewatering / Volume Reduction – belts, presses, centrifuges, beds.
  5. Final Disposal / Beneficial Use – land, landfill, incineration, etc.

1 Thickening Methods

Gravity Thickening

  • Circular tanks w/ picket-scrapers; loading 30!\text{–}60\,\text{kg TS m}^{-2}\,\text{d}^{-1}.
  • SVR (sludge-volume ratio) 0.5!\text{–}20\,\text{d}; blanket 0.5!\text{–}2.5\,\text{m}.
  • Pure primary sludge: 1!\text{–}3\%\rightarrow10\% TS.

Dissolved-Air Flotation (DAF)

  • Air bubbles 60!\text{–}100\,\mu\text{m} attach & float solids.
  • Air-to-solids ratio 0.02\text{:}1!\text{–}0.06\text{:}1.
  • Hydraulic loading 30!\text{–}120\,\text{m}^3\,\text{d}^{-1}\,\text{m}^{-2}.
  • Solids loading 2!\text{–}5\,\text{kg h}^{-1}\,\text{m}^{-2} (increase 50-100 % w/ polymer 2-5 g kg^{-1}).

Mechanical Thickening

  • Centrifuges (solid-bowl, imperforate basket, disc-nozzle) for WAS.
  • Gravity-belt: polymer-flocculated sludge drains on moving fabric; 3!\text{–}7 kg polymer Mg^{-1} DS.
  • Rotary drum: stainless screen cylinder for small–medium plants.

2 Stabilisation / Digestion

Alkaline (Lime) Stabilisation

  • Raise pH >12 for \ge 2\,\text{h} (hydrated \text{Ca(OH)}_2 or quicklime \text{CaO}).
  • Pros: rapid pathogen kill; post-treatment possible.
  • Cons: pH drop over days, temporary; increases solids.

Anaerobic Digestion

  • Multi-stage biochemistry: Hydrolysis → Acidogenesis → Acetogenesis → Methanogenesis.
  • Key microbes: fermenters (Bacteroides, Clostridia), acetogens, methanogens (Archaea).
  • Main methane routes:
    1. Acetoclastic: \text{CH}3\text{COOH} \rightarrow \text{CH}4+\text{CO}_2 (Methanosarcina, Methanosaeta).
    2. Hydrogenotrophic: \text{CO}2+4\text{H}2 \rightarrow \text{CH}4+2\text{H}2\text{O}.
  • Operating envelopes
    • Mesophilic 34!\text{–}36^\circ\text{C}; Thermophilic 50!\text{–}60^\circ\text{C}.
    • pH 6.8!\text{–}7.2; VA/Alk ratio 0.1!\text{–}0.2.
    • Loading \le 3.2\,\text{kg VS m}^{-3}\,\text{d}^{-1}.
  • Configurations
    • Low-rate: no mixing/heating, SRT 30!\text{–}60\,\text{d}.
    • High-rate: heated & mixed, SRT 10!\text{–}20\,\text{d} (often 2-stage).
  • Gas yield 810!\text{–}1120\,\text{L kg}^{-1}\,\text{VS destroyed}; 65!\text{–}69\% CH_4.
  • Tank shapes: cylindrical, German deep cone, egg-shaped.

Aerobic Digestion

  • Oxidises sludge to \text{CO}2, \text{H}2\text{O}, \text{NO}_3^-.
  • Conventional: VSS \le 3\%, SRT 15!\text{–}25\,\text{d}, 200-300 degree-days.
  • Variants
    • ATAD: self-heating to 55!\text{–}60^\circ\text{C} within insulated reactors (3-4 d).
    • High-purity O_2 systems – faster but ↑ cost.
    • Cryophilic <20^\circ\text{C} – needs longer SRT.
  • Pros: simpler, lower capital, odourless product.
  • Cons: high aeration energy; poorer dewatering.

Composting

  • Biological oxidation at 50!\text{–}70^\circ\text{C}; 20!\text{–}30\% VS lost.
  • Mixture moisture optimal 50!\text{–}60\%; bulking agents (wood chips) & amendments (sawdust, straw) add porosity.
  • Process steps: mix → aerate (forced or turning) → curing → screening/reuse bulking agent.
  • PSRP: >40^\circ\text{C} ≥5 d with >55^\circ\text{C} ≥4 h; PFRP stricter.

3 Conditioning for Dewatering

Physical

  • Heat treatment: 177!\text{–}240^\circ\text{C}, 1720!\text{–}2760\,\text{kN m}^{-2}, 15!\text{–}40\,\text{min}.
  • Elutriation: wash digested sludge to remove soluble salts (rare today).

Chemical

  • Inorganics: lime, pebble quicklime, \text{FeCl}_3, alum.
  • Organics: cationic/anionic polymers (polyelectrolytes).
  • Tests: Buchner funnel (specific resistance), CST, jar test.

4 Dewatering Technologies

Belt Filter Press

  • Three zones: chemical conditioning → gravity drainage → wedge/pressure rollers.
  • Cake 18\% TS typical; filtrate returned.

Plate Filter Press

  • Recessed plates form chambers; pump up to 225\,\text{psi} for <2 h.
  • Fixed-volume vs variable-volume (diaphragm squeeze).
  • Produces 35!\text{–}50\% TS cake, low polymer use.

Rotary Vacuum Filter

  • Drum submerged 20-40 %; vacuum 38!\text{–}75\,\text{cm Hg}.
  • Requires chemical conditioning; optimal feed 6-8 % TS.

Centrifuge (Solid-Bowl Decanter)

  • G-force 500!\text{–}1000\,g; cake 15!\text{–}30\% TS.
  • Solids capture 50-80 % (no polymer) up to 95 % (with polymer).

Drying Beds

| Type | Key points |
| Sand (conventional) | 10-23 cm sand over 20-50 cm gravel; seepage + evaporation; thickness ≤15 cm sludge |
| Paved | Concrete/asphalt; 1.5 % slope; drainage or decant variants |
| Artificial media | Wedge-wire or polyurethane panels |
| Vacuum-assisted | Porous plate w/ sub-vacuum chamber; 24-48 h cycle |

Lagoons

  • Long-term (≈18 mo) solar drying; only for well-digested sludge in high-evaporation climates.

5 Ultimate Use & Disposal

  • Beneficial use: agricultural land, forests, parks, golf courses, compost products.
  • Application limited by crop N & P needs; must meet Class A/B & metal standards.
  • Disposal: municipal landfill, monofill, surface disposal piles, incineration.

Numerical / Formula Highlights

  • Air-to-solids (DAF): 0.02!:!0.06.
  • CFR Class A pathogen limit: \text{FC} <10^3\,\text{MPN g}^{-1}\,\text{TS}.
  • Lime: maintain \text{pH}>12 for \ge2\,\text{h} , else re-putrefaction if \text{pH}<11.
  • Anaerobic digester loading: \le3.2\,\text{kg VS m}^{-3}\,\text{d}^{-1}.
  • Egg digester gas: 65!\text{–}69\% CH_4 ideal.
  • Thickener residence (WAS): >18\,\text{h} to limit gas release.
  • Gravity thickener solids loading: 30!\text{–}60\,\text{kg TS m}^{-2}\,\text{d}^{-1}.

Practical / Ethical / Environmental Notes

  • Stabilisation prevents disease transmission (bacteria, viruses, helminths, protozoa, seeds).
  • Vector attraction reduction protects public from flies, mosquitos, rodents.
  • Energy recovery from anaerobic gas → offsets plant power.
  • Lime & heat processes add cost and carbon footprint; trade-offs vs pathogen control.
  • Land application recycles nutrients but must guard against heavy-metal & micro-pollutant buildup.
  • Odour management critical for ATAD and composting (biofilters often installed).